The present invention relates to the deposition of a tertiary, quaternary, pentary, and hexary composite film and specifically to thin film processing and semiconductor thin film processing.
Deposition is one of the basic fabrication processes of modern semiconductor device structures. Deposition techniques include Physical Vapor Deposition (PVD, or sputtering), Chemical Vapor Deposition (CVD), and numerous variations of CVD such as pulsed-CVD, sequential CVD and Atomic Layer Deposition (ALD).
A PVD process uses a high vacuum apparatus and generated plasma to sputter atoms and clusters of atoms from a target toward the surface of a substrate upon which the film is to be deposited. PVD is a line-of-sight deposition process, in that sputtered material from the target tends to move from the target to the substrate and adhere to the first point of contact. This line-of-sight characteristic results in poor coverage on the sidewalls of three-dimensional surface topographies on the wafer surface. Efforts have been made in recent years to improve the conformality of PVD systems but conformal films are generally very difficult to achieve in PVD equipment, especially with aspect ratios of greater than 4:1.
In CVD, a gas or vapor mixture is flowed over the wafer surface at an elevated temperature. Reactions then take place at the hot surface where deposition takes place. The basic characteristic of CVD process is the combined reaction at the substrate of the various precursors in the gas stream. In the case of a mixture of silane and oxygen, for example, silicon from the silane reacts with oxygen to produce silicon dioxide. Excess hydrogen from the silane can either be incorporated into the growing film or pumped away, depending to some extent on the process conditions. The reaction often requires the presence of an energy source such as thermal energy (in the form of resistive heated substrate, or radiative heating) or plasma energy (in the form of plasma excitation). The temperature of the wafer surface is an important factor in the CVD deposition process, because the rate of deposition depends the decomposition and reaction of the precursors, and the reactions rates and decomposition rates are temperature dependent processes. Surface temperature can also affect the uniformity of deposition over the wafer surface. CVD typically requires high temperature for deposition which may not be compatible with other processes in the semiconductor fabrication sequence. CVD at lower temperature tends to produce low quality films in term of film purity, density, and crystallinity. Incomplete reactions, however, at lower temperatures can be enhanced with plasma energy in plasma enhanced CVD process, for example, and by photon energy. CVD technology has been used in semiconductor processing for many years, and the behavior of a wide range of CVD and metalo-organic precursors are understood over wide ranges of temperature. In terms of conformality, CVD processes, much like PVD processes, suffer from poor step coverage over three-dimensional structures on the wafer surface.
Variations of CVD include pulsed-CVD or sequential CVD. In pulsed and sequential CVD, the delivery of the chemical precursors, or the delivery of power from an energy source, is pulsed in such a way that these parameters are not delivered in a continuous manner as in conventional CVD. Pulsing of the chemical precursors can be of one or more of the chemical species required for the growing film. Alternatively, pulsing of the power source can vary energy sources such as plasma energy, thermal energy, laser energy, and photon energy. A major advantage of pulsed-CVD is the potential effect that the transient state resulting from the on-off switching of the precursor or power source has on the process results. Pulsed operational modes are desirable can lead to a reduction in substrate damage and other potentially deleterious effects because of the reduction in delivered power to the substrate with only a modest impact on the reactivity since peak powers can remain the same. The potential reduction in the delivery of precursors for processes in which the delivery of precursors is pulsed is desirable to improve the rate of film growth per volume of incident gas flow, to reduce costs.
Pulsed-CVD can be used to create gradient deposition such as U.S. Pat. No. 5,102,694 of Taylor et al. Taylor discloses a pulsed deposition process in which the precursors are periodically reduced to create a gradient of composition in the deposited films. Taylor's pulsed-CVD relies only on the changing of the first set of precursors to vary the film compositions.
Pulsed-CVD can be used to modulate the precursors flow such as U.S. Pat. No. 5,242,530, entitled “Pulsed gas plasma-enhanced chemical vapor deposition of silicon”, of Batey et al. Batey discloses a pulsed deposition process in which the precursor silane is modulated during a steady flow of plasma hydrogen. The pulsing of silane creates a sequence of deposition steps during the parts of the cycle in which the silane is not flowing, coupled with a sequence of cleaning steps during the parts of the cycle that the silane is not flowing. When silane is not flowing, the flow of plasma-activated hydrogen cleans the surface in preparation for the next cycle of silane gas flow.
Pulsed-CVD can be used to pulse the plasma energy needed for the deposition process such as U.S. Pat. No. 5,344,792, entitled “Pulsed plasma-enhanced CVD of metal silicide conductive films such as TiSi.sub.2”, of Sandhu et al. Sandhu discloses a pulsed deposition process in which the precursors are introduced into a process chamber, then the plasma energy is introduced in pulsed mode to optimize the deposition conditions. U.S. Pat. No. 5,985,375, entitled “Method for pulsed plasma enhanced vapor deposition”, of Donohoe et al. discloses a similar pulsed-CVD process with the plasma energy in pulsed mode but with a power-modulated energy waveform. The pulsing of the plasma energy allows the deposition of a metal film with desired characteristics. U.S. Pat. No. 6,200,651, entitled “Method of chemical vapor deposition in a vacuum plasma processor responsive to a pulsed microwave source”, of Roche et al. discloses a pulsed-CVD process with an electron cyclotron resonance plasma having a repetitive pulsed microwave field to optimize the deposited films. U.S. Pat. No. 6,451,390, entitled “Deposition of TEOS oxide using pulsed RF plasma”, of Goto et al. discloses a TEOS oxide deposition process using a pulsed, RF plasma to control the deposition rate of silicon dioxide. The pulsing feature offers the optimization of the deposited films through the transient state instead of the steady state. Pulsing of plasma during a nitridation process of gate oxide shows less damage than a continuous plasma nitridation process because of increased interactions in the transient plasma state and a reduction in damage due to shorter plasma time.
Pulsed-CVD can be used to pulse the precursors needed for the deposition process such as U.S. Pat. No. 6,306,211, entitled “Method for growing semiconductor film and method for fabricating semiconductor devices”, of Takahashi et al. Takahashi discloses a pulsed-CVD process to deposit epitaxial film of SixGeyCz. Epitaxial deposition requires a single crystal substrate, and the deposited film extends the single crystal nature of the substrate. Epitaxial growth differs from typical CVD in that the films are more typically polycrystalline or amorphous. To extend the single crystal nature of the substrate, the deposited precursors need to bond with the substrate at specific lattice sites, and therefore, low precursor flows are generally preferable in epitaxial deposition to allow the precursors enough time to rearrange into the correct lattice sites. A typical epitaxial process might include a continuous flow of hydrogen to dilute the flow of precursors. Sequential pulses of silicon-based precursor, germanium-based precursor, and carbon-based precursor are then introduced to deposit an epitaxial film of SixGeyCz. To deposit epitaxial films, small amounts of precursors are needed, and the introduction of these small amounts of precursors can be accomplished with short pulses of precursor gases (on the order of micro seconds in duration) and further diluted in high flows of hydrogen. Takahashi discloses that the pulses of the precursors are not overlapped, but is silent on the separation of these pulses. The objective of Takahashi pulsed-CVD is to deposit compound films, therefore the separation of these precursors is not relevant.
Pulsed-CVD as described by Takahashi et al. to deposit epitaxial film of SixGeyCz, does not allow for the deposition of high coverage or conformal film on a non-flat substrate, such as in a via or trench for interconnects or dielectrics in semiconductor devices. The objective of Takahashi pulsed-CVD is to deposit epitaxial films with sufficiently planar surfaces, without mentioning of possible deposition on trenches or vias.
ALD is another variation of CVD using chemical vapor for deposition. In ALD, various gas flows are introduced into a chamber in alternating and separated sequences. For example, a first precursor vapor is delivered into the chamber to be adsorbed on the substrate, the gas flow of this first vapor is then discontinued and residual gases are evacuated from the chamber. Another precursor vapor is then delivered into the chamber to react with the adsorbed molecules on the substrate to form a desired film. The flow of the second precursor gas is then discontinued and residual gases are evacuated from the chamber. This sequence is repeated for many cycles until the deposited film reaches the desired thickness. There are numerous variations of ALD processes, but the ALD processes all share two common characteristics: sequential precursor gas flow and self-limiting thickness per cycle. The sequential precursor flow and evacuation characteristic offers the elimination of gas phase reaction commonly associated with CVD processes. The self-limiting thickness per cycle characteristic offers excellent surface coverage, because the total film thickness does not depend on precursor flow, nor on process time. The total film thickness depends only on the number of cycles. The ALD process then is not sensitive to the substrate temperature. A limitation of ALD is the requirement for reactive sets of paired precursors consisting of a saturating precursor and a reactant. The saturating precursor must adsorb onto the surface of the substrate, and remain bound to the substrate until the delivery of the second reactive precursor to complete the reaction. Non-saturating precursors are better characterized as CVD precursors, since the deposition rate is dependent on substrate temperature and time.
The maximum thickness per cycle in a true ALD process is a single monolayer since the process of saturating the surface is limited by the availability of surface sites. In this process, the precursors do not decompose and do not bond to other precursor gas molecules. The requirement for exposed surface sites for precursor adsorption results in the self-limiting characteristic of ALD and the observed high conformality associated with ALD processes.
Observations of actual ALD processes generally show that the deposition rate is less than a monolayer per cycle as a result of the need to pack large, first precursor molecules together on the surface during the adsorption step. After reaction with the second precursor, a large percentage of the first precursor molecule forms volatile reaction byproducts that are not incorporated into the growing film but rather are pumped away.
Small chamber volumes are typically employed in ALD processing to minimize pumping steps times and fast switching is desirable to minimize transients. The throughput of an ALD process depends on cycle time, which benefits from a small chamber volume. A typical ALD cycle can be as short as a few seconds in duration.
U.S. Pat. No. 5,916,365 to Sherman entitled “Sequential chemical vapor deposition” provides for sequential chemical vapor deposition (ALD) by a sequence of chamber evacuating, adsorption of the first precursor onto the substrate, then another chamber evacuation, then a second radical precursor to react with the adsorbed precursor on the substrate surface, and a third chamber evacuation. The Sherman process produces sub-monolayers per cycle due to the use of the adsorption of saturating first precursors in the adsorption steps. The process cycle can be repeated to grow the desired thickness of film. Sherman discloses an ALD process in which the first precursor process flow is self-limiting, meaning no matter how long the process is, the adsorption thickness cannot be increased. U.S. Pat. No. 6,015,590 to Suntola et al., entitled “Method for growing thin films” discloses an ALD process which completely separates the precursors. Suntola process is an improved ALD process (called ALE by Suntola) meaning the deposition is achieved through the saturation of precursors on the substrate surface and the subsequent reaction with the paired reactants. The advantage of the Suntola process is in the complete separation of precursors, with a better than 99% purging between pulses of precursors to prevent cross reactions.
U.S. Pat. No. 6,200,893, and its divisions (U.S. Pat. Nos. 6,451,695, 6,475,910, U.S. patent publication 2001/0002280, U.S. patent publication 2002/0192954, U.S. patent publication 2002/0197864) to Sneh entitled “Radical-assisted sequential CVD” discuss a method for ALD deposition. The Sneh process sequence is a variation of the ALD process. Sneh discloses a deposition step for the first precursor introduction, but this deposition of Sneh is self-limiting because of the surface saturation with ligands. In U.S. Pat. No. 6,475,910, Sneh discloses a method to extend the thickness of the first precursor introduction step through the addition of another ALD step to increase the thickness of the first precursor introduction step. In a way, this is similar to a nested loop, where the thickness of the first precursor flow step of an ALD process can be increased by another ALD process.